1. Field of the Invention
The present invention relates to an apparatus for amplifying signals, more particularly, to an apparatus for amplifying differential signals.
2. Description of Related Art
In most conditions, the designer of a differential amplifier is eager to increase the frequency bandwidth of the differential amplifier. To increase the bandwidth, a few conventional techniques have been provided. One of the techniques is adding a zero point of the circuit, thereby canceling the original pole. However, the addition of a new zero will generate a new pole so that the design of the circuit becomes more complicated.
Another method is provided in U.S. Pat. No. 6,937,054. In this method, the conductive characteristic of a capacitor device at high frequency is used so that two sets of differential amplifiers are connected in parallel at high frequency to increase the high frequency gain and hence increases the bandwidth. Yet, this method has a number of defects. Because only one set of differential amplifier is in operation at a low frequency, the low frequency gain is sacrificed. Since two sets of amplifiers are connected in parallel at a high frequency, a bigger current is drawn and the capacitor loading at the output terminal is also heavier, which results in limited voltage headroom and considerable capacitive loading. Moreover, due to the parallel connection between two sets of amplifiers and the impedance components connected to either side of one of the amplifiers, overall swing of the output signal is limited.
In the method described in U.S. Pat. No. 6,670,847, inductance is used to cancel the capacitance and increase the bandwidth. One major defect of this method is that the inductor occupies an area far greater than that of the transistor. Another defect of this method is that controlling the characteristic of the inductor remains difficult at the present technological level so that using an inductor requires a longer developing time and a higher production cost.